Mary was not only free from actual sin, but she was also, by a special privilege, cleansed from Original Sin. She had, indeed, to be conceived with, inasmuch as her conception resulted from the commingling of both sexes. For the privilege of conceiving without impairment of virginity was reserved exclusively to her who as a virgin conceived the Son of God. But the commingling of the sexes which, after the sin of our first parent, cannot take place without lust, transmits Original Sin to the offspring. Likewise, if Mary had been conceived without Original Sin, she would not have had to be redeemed by Christ, and so Christ would not be the universal redeemer of men, which detracts from His dignity. Accordingly, we must hold that she was conceived with Original Sin, but was cleansed from it in some special way.
Aquinas’s Shorter Summa
